The New U.S. EPA Method TO-15A Blog Series-Part 5: Humidification

In the previous blog (https://blog.restek.com/the-new-u-s-epa-method-to-15a-blog-series-part4-clean-lines-for-clean-air/) I left off with our system at or under the 20pptv limit required by the new TO-15A method after replacing and moving our air fill line. Once you’re sure of your air quality and the cleanliness of your lines the next step is filling and humidifying your canisters, and if you’ve done water analysis you may be seeing the issue already. The TO-15A analyte list shares a lot of overlap with common volatile contaminants in water, so now that we’ve got our lab air clean it’s time to work on our lab water.

Before I get into what I did here at Restek, let me start with a story. Years ago, I used to work at a lab doing VOC testing in water (methods 8260, 624, and 524), and our quest for clean water was very similar to what we went through to get clean air here. When I started, the DI water system was next to the sample prep lab and was stored in a large tank to keep enough on hand to supply the entire lab. That meant that after being cleaned by the DI system the water sat around and the solvent fumes from the prep lab caused contamination. It was fine for use in the inorganic and semivolatile labs, but to get the blank levels low enough for the VOC lab we had to boil the water down to ¼ of its original volume. Eventually the lab was remodeled and the VOC lab got its own DI system. They also put the VOC lab under positive pressure to keep solvent vapors out from neighboring labs. We could finally stop boiling our water like we were living in the wilderness. We solved our issues by isolating our water from solvent sources, just like I had to with the air here at Restek.

Fortunately for me, Restek does not have a large prep lab going through many liters of solvent per day. Unfortunately, our lab is shared between volatile GC, semivolatile GC, and LC instruments, so I expected some level of contamination to be present. If your lab does VOC testing in water it’s possible you already have good baseline data for your water. If not, then humidifying a canister you know is relatively clean (i.e., used for standards and blanks only, not for field samples, and has been blank tested with dry air) can be a way to evaluate the quality of your water. Using the canister RH calculator that Jason Herrington posted on this blog years ago (https://blog.restek.com/to-15-canister-relative-humidity-part-ii-examples-and-calculations/) I humidified some canisters to 50% RH and tested them. In the same way as the last blog, testing was done on the full TO-15A/NJLL list, but for the sake of simplicity I’m only showing the problematic compounds. The results shown below compare the final results (seven days after initial fill and humidification) for the dry air and humidified air canisters.

Compound Dry (pptv) Humid (pptv)
n-Pentane ND 24
Ethanol ND ND
Acetonitrile 16 25
Carbon disulfide ND ND
Isopropyl alcohol ND ND
Methylene chloride 20 35
Acetone ND 38
Hexane ND 103
Tertiary butanol ND ND
Tetrahydrofuran (THF) 10 889
2-Butanone (MEK) ND 25
Toluene ND 11
4-Methyl-2-2pentanone (MIBK) ND ND

Table 1: Dry vs. Humid air results in pptv

As expected, a number of common solvents are now present. For the most part the levels were lower than I feared, but I was surprised at the high levels of THF as it is not heavily used in our LC applications. However, it is also used as a solvent for resins so it could be coming from our DI system resin cartridges.
Going back to my previous experience, I decided to try boiling the water to see if I could get the results down below 20 pptv. I took some of the DI water and boiled it on a hot plate down to ½ of the original volume, then used it to humidify canisters to 50% RH. Below the table shows the results of the original and boiled water.

Compound Original (pptv) Boiled (pptv)
n-Pentane 24 35
Ethanol ND ND
Acetonitrile 25 24
Carbon disulfide ND ND
Isopropyl alcohol ND ND
Methylene chloride 35 28
Acetone 38 20
Hexane 103 198
Tertiary butanol ND ND
Tetrahydrofuran (THF) 889 39
2-Butanone (MEK) 25 31
Toluene 11 10
4-Methyl-2-2pentanone (MIBK) ND ND

Table 2: Original vs. Boiled water results for humid air in pptv

The boiled water showed great improvement for THF, but mixed results otherwise. Hexane, pentane, and MEK actually seemed to be concentrated during the boiling. Why didn’t it work this time, when I’ve had luck with this technique in the past? Before I was boiling the water further away from the source of solvent contamination, so it’s possible that by boiling it in the same lab as I did here just isn’t practical at low levels. There are a few other things I could try, such as boiling the water in another location or purchasing DI water from a chemical supplier, but those aren’t guaranteed to work and may not be practical for a lot of labs. There’s also the possibility that some of the compounds have always been present in the canisters and they are being displaced from the canister walls by the water vapor, which would mean further chasing clean water would be a red herring.

Despite the handful of compounds above 20 pptv I’m actually rather happy with the result. 20 pptv is a very low limit that I think many labs will have trouble with. It’s likely that in a shared laboratory setting like ours (with ubiquitous solvent use) some level of blank contamination is inevitable, so how can this be handled? Stay tuned for the next blog in the series, where I’ll cover calibrations and how at low levels it’s easy to end up with high biases.

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